The study of the structure of organisms, as well as plants, animals and humans, is the branch of biology called cytology. Scientists have found that the contents of the cell, which is inside it, is quite complex. It is surrounded by the so-called surface apparatus, which includes the outer cell membrane, supra-membrane structures: glycocalyx and microfilaments, pelicule and microtubules that form its submembrane complex.
In this article, we will study the structure and functions of the outer cell membrane, which is part of the surface apparatus various kinds cells.
What are the functions of the outer cell membrane?
As described earlier, the outer membrane is part of the surface apparatus of each cell, which successfully separates its internal contents and protects cell organelles from adverse environmental conditions. Another function is to ensure the exchange of substances between the cell contents and the tissue fluid, therefore, the outer cell membrane transports molecules and ions entering the cytoplasm, and also helps to remove toxins and excess toxic substances from the cell.
The structure of the cell membrane
membranes or plasma membranes various types cells are very different. Mainly, the chemical structure, as well as the relative content of lipids, glycoproteins, proteins in them and, accordingly, the nature of the receptors in them. External which is determined primarily by the individual composition of glycoproteins, takes part in the recognition of environmental stimuli and in the reactions of the cell itself to their actions. Some types of viruses can interact with proteins and glycolipids of cell membranes, as a result of which they penetrate into the cell. Herpes and influenza viruses can use to build their protective shell.
And viruses and bacteria, the so-called bacteriophages, attach to the cell membrane and dissolve it at the point of contact with the help of a special enzyme. Then a molecule of viral DNA passes into the hole formed.
Features of the structure of the plasma membrane of eukaryotes
Recall that the outer cell membrane performs the function of transport, that is, the transfer of substances into and out of it into the external environment. To carry out such a process, a special structure is required. Indeed, the plasmalemma is a constant, universal system of the surface apparatus for all. This is a thin (2-10 Nm), but fairly dense multilayer film that covers the entire cell. Its structure was studied in 1972 by such scientists as D. Singer and G. Nicholson, they also created a fluid-mosaic model of the cell membrane.
The main chemical compounds that form it are ordered molecules of proteins and certain phospholipids, which are interspersed in a liquid lipid environment and resemble a mosaic. Thus, the cell membrane consists of two layers of lipids, the nonpolar hydrophobic "tails" of which are inside the membrane, and the polar hydrophilic heads face the cytoplasm of the cell and the interstitial fluid.
The lipid layer is penetrated by large protein molecules that form hydrophilic pores. It is through them that aqueous solutions of glucose and mineral salts are transported. Some protein molecules are located both on the outer and inner surfaces of the plasmalemma. Thus, on the outer cell membrane in the cells of all organisms with nuclei, there are carbohydrate molecules bound by covalent bonds with glycolipids and glycoproteins. The content of carbohydrates in cell membranes ranges from 2 to 10%.
The structure of the plasmalemma of prokaryotic organisms
The outer cell membrane in prokaryotes performs similar functions to the plasma membranes of cells of nuclear organisms, namely: the perception and transmission of information coming from the external environment, the transport of ions and solutions into and out of the cell, and the protection of the cytoplasm from foreign reagents from the outside. It can form mesosomes - structures that arise when the plasmalemma protrudes into the cell. They may contain enzymes involved in the metabolic reactions of prokaryotes, for example, in DNA replication, protein synthesis.
Mesosomes also contain redox enzymes, while photosynthetics contain bacteriochlorophyll (in bacteria) and phycobilin (in cyanobacteria).
The role of outer membranes in intercellular contacts
Continuing to answer the question of what functions the outer cell membrane performs, let us dwell on its role in plant cells. In plant cells, pores are formed in the walls of the outer cell membrane, passing into the cellulose layer. Through them, the exit of the cytoplasm of the cell to the outside is possible; such thin channels are called plasmodesmata.
Thanks to them, the connection between neighboring plant cells is very strong. In human and animal cells, the sites of contact between adjacent cell membranes are called desmosomes. They are characteristic of endothelial and epithelial cells, and are also found in cardiomyocytes.
Auxiliary formations of the plasmalemma
Understand what is different plant cells from animals, it helps to study the structural features of their plasma membranes, which depend on what functions the outer cell membrane performs. Above it in animal cells is a layer of glycocalyx. It is formed by polysaccharide molecules associated with proteins and lipids of the outer cell membrane. Thanks to the glycocalyx, adhesion (sticking) occurs between cells, leading to the formation of tissues, therefore it takes part in the signaling function of the plasmalemma - the recognition of environmental stimuli.
How is the passive transport of certain substances across cell membranes
As mentioned earlier, the outer cell membrane is involved in the process of transporting substances between the cell and the external environment. There are two types of transport through the plasmalemma: passive (diffusion) and active transport. The first includes diffusion, facilitated diffusion and osmosis. The movement of substances along the concentration gradient depends primarily on the mass and size of the molecules passing through the cell membrane. For example, small non-polar molecules easily dissolve in the middle lipid layer of the plasmalemma, move through it and end up in the cytoplasm.
large molecules organic matter penetrate into the cytoplasm with the help of special carrier proteins. They are species-specific and, when combined with a particle or ion, passively transfer them through the membrane along the concentration gradient without expending energy (passive transport). This process underlies such property of the plasmalemma as selective permeability. In the process, the energy of ATP molecules is not used, and the cell saves it for other metabolic reactions.
Active transport of chemical compounds across the plasmalemma
Since the outer cell membrane ensures the transfer of molecules and ions from the external environment into the cell and back, it becomes possible to remove the products of dissimilation, which are toxins, to the outside, that is, to the intercellular fluid. occurs against a concentration gradient and requires the use of energy in the form of ATP molecules. It also involves carrier proteins called ATPases, which are also enzymes.
An example of such transport is the sodium-potassium pump (sodium ions pass from the cytoplasm to the external environment, and potassium ions are pumped into the cytoplasm). The epithelial cells of the intestine and kidneys are capable of it. Varieties of this method of transfer are the processes of pinocytosis and phagocytosis. Thus, having studied what functions the outer cell membrane performs, it can be established that heterotrophic protists, as well as cells of higher animal organisms, for example, leukocytes, are capable of pino- and phagocytosis.
Bioelectric processes in cell membranes
It has been established that there is a potential difference between the outer surface of the plasmalemma (it is positively charged) and the parietal layer of the cytoplasm, which is negatively charged. It was called the resting potential, and it is inherent in all living cells. And the nervous tissue has not only a resting potential, but is also capable of conducting weak biocurrents, which is called the process of excitation. The outer membranes of nerve cells-neurons, receiving irritation from receptors, begin to change charges: sodium ions massively enter the cell and the surface of the plasmalemma becomes electronegative. And the parietal layer of the cytoplasm, due to an excess of cations, receives a positive charge. This explains why the outer cell membrane of the neuron is recharged, which causes the conduction of nerve impulses that underlie the excitation process.
Nature has created many organisms and cells, but despite this, the structure and most of the functions of biological membranes are the same, which allows us to consider their structure and study their key properties without being tied to a particular type of cell.
What is a membrane?
Membranes are a protective element that is an integral part of the cell of any living organism.
The structural and functional unit of all living organisms on the planet is the cell. Her life is inextricably linked with environment with which it exchanges energy, information, matter. So, the nutritional energy necessary for the functioning of the cell comes from outside and is spent on the implementation of its various functions.
The structure of the simplest structural unit of a living organism: organelle membrane, various inclusions. It is surrounded by a membrane, inside which the nucleus and all organelles are located. These are mitochondria, lysosomes, ribosomes, endoplasmic reticulum. Each structural element has its own membrane.
Role in the life of the cell
The biological membrane plays a culminating role in the structure and functioning of an elementary living system. Only a cell surrounded by a protective shell can rightly be called an organism. A process such as metabolism is also carried out due to the presence of a membrane. If its structural integrity is violated, this leads to a change in the functional state of the organism as a whole.
Cell membrane and its functions
It separates the cytoplasm of the cell from the external environment or from the membrane. The cell membrane ensures the proper performance of specific functions, the specifics of intercellular contacts and immune manifestations, and supports the transmembrane difference in electrical potential. It contains receptors that can perceive chemical signals - hormones, mediators and other biologically active components. These receptors give it another ability - to change the metabolic activity of the cell.
Membrane functions:
1. Active transfer of substances.
2. Passive transfer of substances:
2.1. Diffusion is simple.
2.2. transport through the pores.
2.3. Transport carried out by diffusion of a carrier along with a membrane substance or by relaying a substance along the molecular chain of a carrier.
3. Transfer of non-electrolytes due to simple and facilitated diffusion.
The structure of the cell membrane
The components of the cell membrane are lipids and proteins.
Lipids: phospholipids, phosphatidylethanolamine, sphingomyelin, phosphatidylinositol and phosphatidylserine, glycolipids. The proportion of lipids is 40-90%.
Proteins: peripheral, integral (glycoproteins), spectrin, actin, cytoskeleton.
The main structural element is a double layer of phospholipid molecules.
Roof membrane: definition and typology
Some statistics. In the territory Russian Federation membrane as a roofing material has been used not so long ago. The share of membrane roofs from the total number of soft roof slabs is only 1.5%. Bituminous and mastic roofs have become more widespread in Russia. But in Western Europe, membrane roofs account for 87%. The difference is palpable.
As a rule, the membrane as the main material in the roof overlap is ideal for flat roofs. For those with a large bias, it is less suitable.
The volumes of production and sales of membrane roofs in the domestic market have a positive growth trend. Why? The reasons are more than clear:
- The service life is about 60 years. Imagine, only the warranty period of use, which is set by the manufacturer, reaches 20 years.
- Ease of installation. For comparison: the installation of a bituminous roof takes 1.5 times more time than the installation of a membrane floor.
- Ease of maintenance and operation repair work.
The thickness of roofing membranes can be 0.8-2 mm, and the average weight of one square meter is 1.3 kg.
Properties of roofing membranes:
- elasticity;
- strength;
- resistance to ultraviolet rays and other aggressor media;
- frost resistance;
- fire resistance.
There are three types of roofing membrane. The main classification feature is the species polymer material constituting the base of the canvas. So, roofing membranes are:
- belonging to the EPDM group, are made on the basis of polymerized ethylene-propylene-diene monomer, in other words, Advantages: high strength, elasticity, water resistance, environmental friendliness, low cost. Disadvantages: adhesive technology for joining canvases by using a special tape, low strength of the joints. Scope of application: used as a waterproofing material for tunnel ceilings, water sources, waste storages, artificial and natural reservoirs, etc.
- PVC membranes. These are shells, in the production of which polyvinyl chloride is used as the main material. Advantages: UV resistance, fire resistance, extensive color range of membrane sheets. Disadvantages: low resistance to bituminous materials, oils, solvents; emits harmful substances into the atmosphere; the color of the canvas fades over time.
- TPO. Made from thermoplastic olefins. They can be reinforced and non-reinforced. The first are equipped with a polyester mesh or fiberglass cloth. Advantages: environmental friendliness, durability, high elasticity, temperature resistance (both at high and low temperatures), welded joints of the seams of the canvases. Disadvantages: high price category, lack of manufacturers in the domestic market.
Profiled membrane: characteristics, functions and benefits
Profiled membranes are an innovation in the construction market. Such a membrane is used as a waterproofing material.
The material used in the manufacture is polyethylene. The latter is of two types: high pressure polyethylene (LDPE) and low pressure polyethylene (HDPE).
Indicator | ||
Tensile strength (MPa) | ||
Tensile elongation (%) | ||
Density (kg / m3) | ||
Compressive strength (MPa) | ||
Impact strength (notched) (KJ/sqm) | ||
Flexural modulus (MPa) | ||
Hardness (MPa) | ||
Operating temperature (˚С) | -60 to +80 | -60 to +80 |
Daily rate of water absorption (%) |
The profiled membrane made of high pressure polyethylene has a special surface - hollow pimples. The height of these formations can vary from 7 to 20 mm. Inner surface membranes are smooth. This enables trouble-free bending of building materials.
A change in the shape of individual sections of the membrane is excluded, since the pressure is evenly distributed over its entire area due to the presence of all the same protrusions. Geomembrane can be used as ventilation insulation. In this case, free heat exchange inside the building is ensured.
Benefits of profiled membranes:
- increased strength;
- heat resistance;
- stability of chemical and biological influence;
- long service life (more than 50 years);
- ease of installation and maintenance;
- affordable cost.
Profiled membranes are of three types:
- with a single layer;
- with a two-layer canvas = geotextile + drainage membrane;
- with a three-layer canvas = slippery surface + geotextile + drainage membrane.
A single-layer profiled membrane is used to protect the main waterproofing, installation and dismantling of the preparation of concrete walls with high humidity. A two-layer protective one is used during equipment. A three-layer one is used on soil that lends itself to frost heaving and deep soil.
Areas of use for drainage membranes
The profiled membrane finds its application in the following areas:
- Basic foundation waterproofing. Provides reliable protection against damaging influences ground water, root systems of plants, soil subsidence, mechanical damage.
- Foundation wall drainage. Neutralizes the impact of groundwater, precipitation by transferring them to drainage systems.
- Horizontal type - protection against deformation due to structural features.
- An analogue of concrete preparation. Operated in case of construction works for the construction of buildings in the zone of low groundwater, in cases where horizontal waterproofing is used to protect against capillary moisture. Also, the functions of the profiled membrane include the impermeability of cement laitance into the soil.
- Ventilation of wall surfaces advanced level humidity. It can be installed both on the inside and on the outside of the room. In the first case, air circulation is activated, and in the second, optimal humidity and temperature are ensured.
- Used inverted roof.
Super diffusion membrane
The superdiffusion membrane is a material of a new generation, the main purpose of which is to protect the elements of the roof structure from wind phenomena, precipitation, and steam.
The production of protective material is based on the use of nonwovens, high quality dense fibers. In the domestic market, a three-layer and four-layer membrane is popular. Reviews of experts and consumers confirm that the more layers underlie the design, the stronger its protective functions, and therefore the higher the energy efficiency of the room as a whole.
Depending on the type of roof, the features of its design, climatic conditions, manufacturers recommend giving preference to one or another type of diffusion membranes. So, they exist for pitched roofs of complex and simple structures, for pitched roofs with a minimum slope, for folded roofs, etc.
The superdiffusion membrane is laid directly on the heat-insulating layer, flooring from the boards. There is no need for a ventilation gap. The material is fastened with special brackets or steel nails. The edges of the diffusion sheets are connected. Work can be carried out even under extreme conditions: in strong gusts of wind, etc.
In addition, the coating in question can be used as a temporary roof covering.
PVC membranes: essence and purpose
PVC membranes are a roofing material made from polyvinyl chloride and have elastic properties. Such a modern roofing material completely replaced bituminous roll analogues, which have a significant drawback - the need for systematic maintenance and repair. To date characteristics PVC membranes allow them to be used when carrying out repair work on old flat roofs. They are also used when installing new roofs.
A roof made of such material is easy to use, and its installation is possible on any type of surface, at any time of the year and under any weather conditions. PVC membrane has the following properties:
- strength;
- stability when exposed to UV rays, various types of precipitation, point and surface loads.
It is thanks to their unique properties PVC membranes will serve you faithfully for many years. The life of such a roof is equal to the life of the building itself, while roll roofing materials need regular repairs, and in some cases even dismantling and installing a new floor.
Between themselves, PVC membrane sheets are connected by hot breath welding, the temperature of which is in the range of 400-600 degrees Celsius. This connection is completely sealed.
Advantages of PVC membranes
Their advantages are obvious:
- the flexibility of the roofing system, which is most consistent with the construction project;
- durable, airtight connecting seam between the membrane sheets;
- ideal tolerance to climate change, weather conditions, temperature, humidity;
- increased vapor permeability, which contributes to the evaporation of moisture accumulated in the under-roof space;
- many color options;
- fire-fighting properties;
- the ability to maintain the original properties and appearance for a long period;
- PVC membrane - absolutely eco-friendly material, which is confirmed by the relevant certificates;
- the installation process is mechanized, so it will not take much time;
- operating rules allow the installation of various architectural additions directly on top of the PVC membrane roof itself;
- single-layer styling will save you money;
- ease of maintenance and repair.
Membrane fabric
Membrane fabric has been known to the textile industry for a long time. Shoes and clothes are made from this material: for adults and children. Membrane - the basis of membrane fabric, presented in the form of a thin polymer film and having such characteristics as water resistance and vapor permeability. For the production of this material, this film is covered with outer and inner protective layers. Their structure is determined by the membrane itself. This is done in order to save all useful properties even if damaged. In other words, membrane clothing does not get wet when exposed to precipitation in the form of snow or rain, but at the same time it perfectly passes steam from the body into the external environment. This throughput allows the skin to breathe.
Considering all of the above, we can conclude that ideal winter clothes are made from such a fabric. The membrane, which is at the base of the fabric, can be:
- with pores;
- without pores;
- combined.
Teflon is included in the composition of membranes with many micropores. The dimensions of such pores do not even reach the dimensions of a drop of water, but are larger than a water molecule, which indicates water resistance and the ability to remove sweat.
Membranes that do not have pores are usually made from polyurethane. Their inner layer concentrates all sweat-fat secretions of the human body and pushes them out.
The structure of the combined membrane implies the presence of two layers: porous and smooth. This fabric has high quality characteristics and will last for many years.
Thanks to these advantages, clothes and shoes made of membrane fabrics and designed to be worn in the winter season are durable, but light, and perfectly protect against frost, moisture, and dust. They are simply indispensable for many active types of winter recreation, mountaineering.
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Cells are separated from the internal environment of the body by a cell or plasma membrane.
The membrane provides:
1) Selective penetration into and out of the cell of molecules and ions necessary to perform specific cell functions;
2) Selective transport of ions across the membrane, maintaining a transmembrane electric potential difference;
3) The specifics of intercellular contacts.
Due to the presence in the membrane of numerous receptors that perceive chemical signals - hormones, mediators and other biologically active substances, it is able to change the metabolic activity of the cell. Membranes provide the specificity of immune manifestations due to the presence of antigens on them - structures that cause the formation of antibodies that can specifically bind to these antigens.
The nucleus and organelles of the cell are also separated from the cytoplasm by membranes that prevent the free movement of water and substances dissolved in it from the cytoplasm to them and vice versa. This creates conditions for the separation of biochemical processes occurring in different compartments (compartments) inside the cell.
cell membrane structure
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The cell membrane is an elastic structure, with a thickness of 7 to 11 nm (Fig. 1.1). It consists mainly of lipids and proteins. From 40 to 90% of all lipids are phospholipids - phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin and phosphatidylinositol. An important component of the membrane are glycolipids, represented by cerebrosides, sulfatides, gangliosides and cholesterol.
Rice. 1.1 Organization of the membrane.
The main structure of the cell membrane is a double layer of phospholipid molecules. Due to hydrophobic interactions, the carbohydrate chains of lipid molecules are held near each other in an extended state. Groups of phospholipid molecules of both layers interact with protein molecules immersed in the lipid membrane. Due to the fact that most of the lipid components of the bilayer are in a liquid state, the membrane has mobility and undulates. Its sections, as well as proteins immersed in the lipid bilayer, will mix from one part to another. Mobility (fluidity) of cell membranes facilitates the transport of substances through the membrane.
cell membrane proteins represented mainly by glycoproteins. Distinguish:
integral proteins penetrating through the entire thickness of the membrane and
peripheral proteins attached only to the surface of the membrane, mainly to its inner part.
Peripheral proteins almost all function as enzymes (acetylcholinesterase, acid and alkaline phosphatases, etc.). But some enzymes are also represented by integral proteins - ATPase.
integral proteins provide a selective exchange of ions through the membrane channels between the extracellular and intracellular fluid, and also act as proteins - carriers of large molecules.
Membrane receptors and antigens can be represented by both integral and peripheral proteins.
Proteins adjacent to the membrane from the cytoplasmic side belong to cell cytoskeleton . They can attach to membrane proteins.
So, protein strip 3 (band number during protein electrophoresis) of erythrocyte membranes is combined into an ensemble with other cytoskeleton molecules - spectrin through the low molecular weight protein ankyrin (Fig. 1.2).
Rice. 1.2 Scheme of the arrangement of proteins in the membrane cytoskeleton of erythrocytes.1 - spectrin; 2 - ankyrin; 3 - protein band 3; 4 - protein band 4.1; 5 - protein band 4.9; 6 - actin oligomer; 7 - protein 6; 8 - gpicophorin A; 9 - membrane.
Spectrin is the main protein of the cytoskeleton, constituting a two-dimensional network to which actin is attached.
actin forms microfilaments, which are the contractile apparatus of the cytoskeleton.
cytoskeleton allows the cell to exhibit flexibly elastic properties, provides additional strength to the membrane.
Most integral proteins are glycoproteins. Their carbohydrate part protrudes from the cell membrane to the outside. Many glycoproteins have a large negative charge due to the significant content of sialic acid (for example, the glycophorin molecule). This provides the surface of most cells with a negative charge, helping to repel other negatively charged objects. Carbohydrate protrusions of glycoproteins carry blood group antigens, other antigenic determinants of the cell, and act as hormone-binding receptors. Glycoproteins form adhesive molecules that cause cells to attach to each other, i.e. close intercellular contacts.
Features of metabolism in the membrane
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Membrane components are subject to many metabolic transformations under the influence of enzymes located on their membrane or inside it. These include oxidative enzymes that play an important role in modifying the hydrophobic elements of membranes - cholesterol, etc. In membranes, when enzymes - phospholipases are activated, biologically active compounds - prostaglandins and their derivatives - are formed from arachidonic acid. As a result of the activation of phospholipid metabolism in the membrane, thromboxanes and leukotrienes are formed, which have a powerful effect on platelet adhesion, inflammation, etc.
The membrane constantly undergoes renewal processes of its components. . Thus, the lifetime of membrane proteins ranges from 2 to 5 days. However, there are mechanisms in the cell that ensure the delivery of newly synthesized protein molecules to membrane receptors, which facilitate the incorporation of the protein into the membrane. The "recognition" of this receptor by the newly synthesized protein is facilitated by the formation of a signal peptide, which helps to find the receptor on the membrane.
Membrane lipids also have a significant metabolic rate., which requires for the synthesis of these membrane components a large number fatty acids.
The specifics of the lipid composition of cell membranes are affected by changes in the human environment and the nature of his diet.
For example, an increase in dietary fatty acids with unsaturated bonds increases the liquid state of lipids in cell membranes of various tissues, leads to a change in the ratio of phospholipids to sphingomyelins and lipids to proteins that is favorable for the function of the cell membrane.
Excess cholesterol in membranes, on the contrary, increases the microviscosity of their bilayer of phospholipid molecules, reducing the rate of diffusion of certain substances through cell membranes.
Food enriched with vitamins A, E, C, P improves lipid metabolism in erythrocyte membranes, reduces membrane microviscosity. This increases the deformability of erythrocytes, facilitates their transport function (Chapter 6).
Deficiency of fatty acids and cholesterol in food disrupts the lipid composition and function of cell membranes.
For example, a fat deficiency disrupts the function of the neutrophil membrane, which inhibits their ability to move and phagocytosis (active capture and absorption of microscopic foreign living objects and solid particles by unicellular organisms or some cells).
In the regulation of the lipid composition of membranes and their permeability, regulation of cell proliferation an important role is played by reactive oxygen species, which are formed in the cell in conjunction with normal metabolic reactions (microsomal oxidation, etc.).
Formed reactive oxygen species- superoxide radical (O 2), hydrogen peroxide (H 2 O 2), etc. are extremely reactive substances. Their main substrate in free radical oxidation reactions are unsaturated fatty acids that are part of cell membrane phospholipids (the so-called lipid peroxidation reactions). The intensification of these reactions can cause damage to the cell membrane, its barrier, receptor and metabolic functions, modification of nucleic acid molecules and proteins, which leads to mutations and inactivation of enzymes.
Under physiological conditions, the intensification of lipid peroxidation is regulated by the antioxidant system of cells, represented by enzymes that inactivate reactive oxygen species - superoxide dismutase, catalase, peroxidase and substances with antioxidant activity - tocopherol (vitamin E), ubiquinone, etc. A pronounced protective effect on cell membranes (cytoprotective effect) with various damaging effects on the body, prostaglandins E and J2 have, "extinguishing" the activation of free radical oxidation. Prostaglandins protect the gastric mucosa and hepatocytes from chemical damage, neurons, neuroglial cells, cardiomyocytes - from hypoxic damage, skeletal muscles- with severe physical activity. Prostaglandins, binding to specific receptors on cell membranes, stabilize the bilayer of the latter, reduce the loss of phospholipids by membranes.
Membrane receptor functions
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A chemical or mechanical signal is first perceived by cell membrane receptors. The consequence of this is the chemical modification of membrane proteins, which leads to the activation of "second messengers" that ensure the rapid propagation of the signal in the cell to its genome, enzymes, contractile elements, etc.
Schematically, transmembrane signaling in a cell can be represented as follows:
1) Excited by the perceived signal, the receptor activates the γ-proteins of the cell membrane. This occurs when they bind guanosine triphosphate (GTP).
2) The interaction of the "GTP-y-proteins" complex, in turn, activates the enzyme - the precursor of secondary messengers, located on inside membranes.
The precursor of one secondary messenger - cAMP, formed from ATP, is the enzyme adenylate cyclase;
The precursor of other secondary messengers - inositol triphosphate and diacylglycerol, formed from membrane phosphatidylinositol-4,5-diphosphate, is the enzyme phospholipase C. In addition, inositol triphosphate mobilizes another secondary messenger in the cell - calcium ions, which are involved in almost all regulatory processes in the cell. For example, the resulting inositol triphosphate causes the release of calcium from the endoplasmic reticulum and an increase in its concentration in the cytoplasm, thereby including various forms of cellular response. With the help of inositol triphosphate and diacylglycerol, the function of smooth muscles and B-cells of the pancreas is regulated by acetylcholine, the anterior pituitary thyropin-releasing factor, the response of lymphocytes to antigen, etc.
In some cells, the role of the second messenger is performed by cGMP, which is formed from GTP with the help of the enzyme guanylate cyclase. It serves, for example, as a second messenger for natriuretic hormone in the smooth muscle of blood vessel walls. cAMP serves as a second messenger for many hormones - adrenaline, erythropoietin, etc. (Chapter 3).
The membrane is a hyperfine structure that forms the surface of organelles and the cell as a whole. All membranes have a similar structure and are connected in one system.
Chemical composition
Cell membranes are chemically homogeneous and consist of proteins and lipids of various groups:
- phospholipids;
- galactolipids;
- sulfolipids.
They also contain nucleic acids, polysaccharides and other substances.
Physical Properties
At normal temperature, the membranes are in a liquid-crystalline state and constantly fluctuate. Their viscosity is close to that of vegetable oil.
The membrane is recoverable, strong, elastic and has pores. The thickness of the membranes is 7 - 14 nm.
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For large molecules, the membrane is impermeable. Small molecules and ions can pass through the pores and the membrane itself under the influence of the concentration difference on different sides of the membrane, as well as with the help of transport proteins.
Model
The structure of membranes is usually described using a fluid mosaic model. The membrane has a frame - two rows of lipid molecules, tightly, like bricks, adjacent to each other.
Rice. 1. Sandwich-type biological membrane.
On both sides, the surface of lipids is covered with proteins. The mosaic pattern is formed by protein molecules unevenly distributed on the surface of the membrane.
According to the degree of immersion in the bilipid layer, protein molecules are divided into three groups:
- transmembrane;
- submerged;
- superficial.
Proteins provide the main property of the membrane - its selective permeability for various substances.
Membrane types
All cell membranes according to localization can be divided into the following types:
- outdoor;
- nuclear;
- organelle membranes.
The outer cytoplasmic membrane, or plasmolemma, is the boundary of the cell. Connecting with elements of the cytoskeleton, it maintains its shape and size.
Rice. 2. Cytoskeleton.
The nuclear membrane, or karyolemma, is the boundary of the nuclear content. It is built from two membranes, very similar to the outer one. The outer membrane of the nucleus is connected to the membranes of the endoplasmic reticulum (ER) and, through pores, to the inner membrane.
EPS membranes penetrate the entire cytoplasm, forming surfaces on which various substances are synthesized, including membrane proteins.
Organoid membranes
Most organelles have a membrane structure.
Walls are built from one membrane:
- Golgi complex;
- vacuoles;
- lysosomes.
Plastids and mitochondria are built from two layers of membranes. Their outer membrane is smooth, and the inner one forms many folds.
Features of the photosynthetic membranes of chloroplasts are embedded chlorophyll molecules.
Animal cells have a carbohydrate layer called the glycocalyx on the surface of the outer membrane.
Rice. 3. Glycocalyx.
The glycocalyx is most developed in the cells of the intestinal epithelium, where it creates conditions for digestion and protects the plasmolemma.
Table "Structure of the cell membrane"
What have we learned?
We examined the structure and functions of the cell membrane. The membrane is a selective (selective) barrier of the cell, nucleus and organelles. The structure of the cell membrane is described by a fluid-mosaic model. According to this model, protein molecules are embedded in a double layer of viscous lipids.
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